Science: Atoms and Molecules: Molar Mass and Writing Chemical Formulae of Compounds (For CBSE, ICSE, IAS, NET, NRA 2022)

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1 mol of hydrogen atoms = 6.022 x 1023

1 mol of water molecule = 6.022 x 1023

1 mol of sodium chloride = 6.022 × 1023 units of NaCl.

1 mole of various substances

Molar Mass

  • The mass of one mole of a substance in grams is called its molar mass.
  • The molar mass in grams is numerically equal to atomic/molecular/formula mass in u.

Molar mass of oxygen (O) = 16.02g.

Molar mass of water (H2O) = 18.02g.

Molar mass of carbon (C) = 12g.

Molar mass of sodium chloride (NaCl) = 58.5g.

Writing Chemical Formulae of Compounds

The formula of a compound indicates:

  • elements constituting the compound and
  • the number of each constituent element. In other word, the formula of a compound also represents its chemical composition.

Valency and Formulation

  • Most of the simple compounds are made of two elements. Such compounds are called binary compounds.
  • It is easy to write formula of such compounds. When a metal combines with a non-metal, the symbol of the metal-element is written on the left-hand side and that of the non-metal element on right hand side.
Valency and Formulation
Valency and Formulation
ElementsSymbolValencyElementsSymbolValency
HydrogenH1PhosphorusP5
OxygenO2SodiumNa1
CarbonC4MagnesiumMg2
NitrogenN3CalciumCa2
ChlorineCl1AluminumAl3
BromineBr1IronFe2
IodineI1BariumBa2

Thus, we can write formulas of various compounds if we know elements constituting them and their valencies.

Formulation of Ionic Compounds

Formulation of an ionic compound is easy when we know charge of cation and anion. Remember, in an ionic compound, sum of the charge of cation and anion should be equal to zero . Suppose you have to write formula of sodium sulphate which is made of Na + and SO42 – ions. For this the positive and negative charge can be crossed over to give subscripts. The purpose of this crossing over of charges is to find the number of ions required to equate the number of positive and negative charges.

Formulation of Ionic Compounds

This gives the formula of sodium sulphate as Na2SO4. We can check the charge balance as follows:

Formulation of Ionic Compounds

Thus, the compound, Na2SO4 is electrically neutral.

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